41 research outputs found
Orgin of Subgap Features in Transparent Amorphous Oxide Semiconductors
Amorphous indium gallium zinc oxide (a-IGZO) is a transparent amorphous oxide semiconductor (TAOS) that has shown promise as transparent thin film transistors (TTFTs) in various display applications. Within a-IGZO films, states can form in the band gap that hinder TTFT performance. In order to elucidate the origin of these states, we examined the formation of the subgap states for a-IGZO thin films with various compositions. We observed a positive correlation between the subgap formation and the percentage of oxygen ruling out oxygen deficiencies as a factor in the formation of subgap states. Furthermore, metallic indium crystallites were observed to form for samples grown in oxygen deficient conditions. Our studies reveal that subgap states are due to oxygen with few surrounding metal neighbors and indium with few surrounding oxygen neighbors. The subgap states can be suppressed in a-IGZO (and other TAOS) by careful choice of composition
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Dissociate lattice oxygen redox reactions from capacity and voltage drops of battery electrodes.
The oxygen redox (OR) activity is conventionally considered detrimental to the stability and kinetics of batteries. However, OR reactions are often confused by irreversible oxygen oxidation. Here, based on high-efficiency mapping of resonant inelastic x-ray scattering of both the transition metal and oxygen, we distinguish the lattice OR in Na0.6[Li0.2Mn0.8]O2 and compare it with Na2/3[Mg1/3Mn2/3]O2. Both systems display strong lattice OR activities but with distinct electrochemical stability. The comparison shows that the substantial capacity drop in Na0.6[Li0.2Mn0.8]O2 stems from non-lattice oxygen oxidations, and its voltage decay from an increasing Mn redox contribution upon cycling, contrasting those in Na2/3[Mg1/3Mn2/3]O2. We conclude that lattice OR is not the ringleader of the stability issue. Instead, irreversible oxygen oxidation and the changing cationic reactions lead to the capacity and voltage fade. We argue that lattice OR and other oxygen activities should/could be studied and treated separately to achieve viable OR-based electrodes
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How Bulk Sensitive is Hard X-ray Photoelectron Spectroscopy: Accounting for the Cathode-Electrolyte Interface when Addressing Oxygen Redox.
Sensitivity to the "bulk" oxygen core orbital makes hard X-ray photoelectron spectroscopy (HAXPES) an appealing technique for studying oxygen redox candidates. Various studies have reported an additional O 1s peak (530-531 eV) at high voltages, which has been considered a direct signature of the bulk oxygen redox process. Here, we find the emergence of a 530.4 eV O 1s HAXPES peak for three model cathodes-Li2MnO3, Li-rich NMC, and NMC 442-that shows no clear link to oxygen redox. Instead, the 530.4 eV peak for these three systems is attributed to transition metal reduction and electrolyte decomposition in the near-surface region. Claims of oxygen redox relying on photoelectron spectroscopy must explicitly account for the surface sensitivity of this technique and the extent of the cathode degradation layer
Accelerated optimization of transparent, amorphous zinc-tin-oxide thin films for optoelectronic applications
In the last decade, transparent amorphous oxide semiconductors (TAOS) have become an essential component of many electronics, from ultra high resolution displays to solar cells. However, these disordered oxides typically rely on expensive component metals like indium to provide sufficient charge carrier conduction, and their optoelectronic properties are not as predictable and well-described as those of traditional, crystalline semiconductors. Herein we report on our comprehensive study of the amorphous zinc-tin-oxide (a-ZTO) system for use as an indium-free, n-type TAOS. Using a combination of high-throughput co-deposition growth, high resolution spectral mapping, and atomistic calculations, we explain the development of disorder-related subgap states in SnO2-like a-ZTO and optical bandgap reduction in ZnO-like a-ZTO. In addition, we report on a composition-induced electronic and structural transition in ZnO-like a-ZTO resulting in an exceptionally high figure of merit, comparable to that of amorphous indium-gallium-zinc-oxide. Our results accelerate the development of a-ZTO and similar systems as indium-free TAOS materials
Influence of Polymorphism on the Electronic Structure of Ga2O3
The search for new wide band gap materials is intensifying to satisfy the
need for more advanced and energy efficient power electronic devices.
GaO has emerged as an alternative to SiC and GaN, sparking a renewed
interest in its fundamental properties beyond the main -phase. Here,
three polymorphs of GaO, , and , are
investigated using X-ray diffraction, X-ray photoelectron and absorption
spectroscopy, and ab initio theoretical approaches to gain insights into their
structure - electronic structure relationships. Valence and conduction
electronic structure as well as semi-core and core states are probed, providing
a complete picture of the influence of local coordination environments on the
electronic structure. State-of-the-art electronic structure theory, including
all-electron density functional theory and many-body perturbation theory,
provide detailed understanding of the spectroscopic results. The calculated
spectra provide very accurate descriptions of all experimental spectra and
additionally illuminate the origin of observed spectral features. This work
provides a strong basis for the exploration of the GaO polymorphs as
materials at the heart of future electronic device generations.Comment: Updated manuscript version after peer revie
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Surface Chemistry Dependence on Aluminum Doping in Ni-rich LiNi 0.8 Co 0.2â y Al y O 2 Cathodes
Abstract: Aluminum is a common dopant across oxide cathodes for improving the bulk and cathode-electrolyte interface (CEI) stability. Aluminum in the bulk is known to enhance structural and thermal stability, yet the exact influence of aluminum at the CEI remains unclear. To address this, we utilized a combination of X-ray photoelectron and absorption spectroscopy to identify aluminum surface environments and extent of transition metal reduction for Ni-rich LiNi0.8Co0.2âyAlyO2 (0%, 5%, or 20% Al) layered oxide cathodes tested at 4.75 V under thermal stress (60 °C). For these tests, we compared the conventional LiPF6 salt with the more thermally stable LiBF4 salt. The CEI layers are inherently different between these two electrolyte salts, particularly for the highest level of Al-doping (20%) where a thicker (thinner) CEI layer is found for LiPF6 (LiBF4). Focusing on the aluminum environment, we reveal the type of surface aluminum species are dependent on the electrolyte salt, as Al-O-F- and Al-F-like species form when using LiPF6 and LiBF4, respectively. In both cases, we find cathode-electrolyte reactions drive the formation of a protective Al-F-like barrier at the CEI in Al-doped oxide cathodes
Lone-pair stabilization in transparent amorphous tin oxides:a potential route to p-type conduction pathways
The electronic and atomic structures of amorphous transparent tin oxides have been investigated by a combination of X-ray spectroscopy and atomistic calculations. Crystalline SnO is a promising p-type transparent oxide semiconductor due to a complex lone-pair hybridization that affords both optical transparency despite a small electronic band gap and spherical s-orbital character at the valence band edge. We find that both of these desirable properties (transparency and s-orbital valence band character) are retained upon amorphization despite the disruption of the layered lone-pair states by structural disorder. We explain the anomalously large band gap widening necessary to maintain transparency in terms of lone-pair stabilization via atomic clustering. Our understanding of this mechanism suggests that continuous hole conduction pathways along extended lone pair clusters should be possible under certain stoichiometries. Moreover, these findings should be applicable to other lone-pair active semiconductors
Resonant doping for high mobility transparent conductors: the case of Mo-doped In2O3
Transparent conductors are a vital component of smartphones, touch-enabled displays, low emissivity windows and thin film photovoltaics. Tin-doped In2O3 (ITO) dominates the transparent conductive films market, accounting for the majority of the current multi-billion dollar annual global sales. Due to the high cost of indium, however, alternatives to ITO have been sought but have inferior properties. Here we demonstrate that molybdenum-doped In2O3 (IMO) has higher mobility and therefore higher conductivity than ITO with the same carrier density. This also results in IMO having increased infrared transparency compared to ITO of the same conductivity. These properties enable current performance to be achieved using thinner films, reducing the amount of indium required and raw material costs by half. The enhanced doping behavior arises from Mo 4d donor states being resonant high in the conduction band and negligibly perturbing the host conduction band minimum, in contrast to the adverse perturbation caused by Sn 5s dopant states. This new understanding will enable better and cheaper TCOs based on both In2O3 and other metal oxides